Intergranular M23C6 Carbide Precipitation Behavior and Its Effect on Mechanical Properties of Inconel 690 Tubes

Abstract

The precipitation behaviors of intergranular carbides on various types of grain boundaries and their effect on tensile properties at room temperature were investigated systematically in Inconel 690 tubes. Precipitates of intergranular chromium carbide were identified as Cr-rich M23C6. It was observed that the M23C6 carbide behavior is obviously influenced by grain boundary character and interfacial energy. The intergranular M23C6 carbides were suppressed on low-angle boundaries and coincidence-site lattice boundaries. Plenty of M23C6 carbides precipitated at random high-angle boundaries with high interfacial energy. With increasing aging time, the precipitated carbides became coarse and grew into the matrix near grain boundaries. The ultimate tensile strength and elongation of the Inconel 690 tube solution annealed at 1373 K (1100 °C) for 15 minutes were 671.2 MPa and 47.2 pct, respectively. After aging treatment, the ultimate tensile strength and elongation drastically decreased with increasing aging time. The tensile test generated a number of cracks at the grain boundaries due to coarse intergranular M23C6 carbides near the fracture. Both the decrease of ultimate tensile strength and elongation of the aged Inconel 690 tube were mainly caused by breaking of the coarse M23C6 carbide and reduced chromium concentration in the matrix.